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1. Overview: The Flow of Genetic Information

1. Overview: The Flow of Genetic Information. The information content of DNA is in the form of specific sequences of nucleotides The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins

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1. Overview: The Flow of Genetic Information

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  1. 1. Overview: The Flow of Genetic Information • The information content of DNA is in the form of specific sequences of nucleotides • The DNA inherited by an organism leads to specific traits by dictating the synthesis of proteins • Gene expression, the process by which DNA directs protein synthesis, includes two stages: transcription and translation • The ribosome is part of the cellular machinery for translation, polypeptide synthesis

  2. Concept 17.1: Genes specify proteins via transcription and translation • How was the fundamental relationship between genes and proteins discovered?

  3. 2—3. Evidence from the Study of Metabolic Defects • In 1909, British physician Archibald Garrod first suggested that genes dictate phenotypes through enzymes that catalyze specific chemical reactions • He thought symptoms of an inherited disease reflect an inability to synthesize a certain enzyme • (ex. alkaptonuria) • Linking genes to enzymes required understanding that cells synthesize and degrade molecules in a series of steps, a metabolic pathway

  4. 4. • George Beadle proposed a “one gene–one enzyme” hypothesis, which states that each gene dictates production of a specific enzyme

  5. 5—7. Nutritional Mutants in Neurospora: Scientific Inquiry • Beadle and Tatum exposed bread mold to X-rays, creating mutants that were unable to survive on minimal medium as a result of inability to synthesize certain molecules • Using crosses, they identified three classes of arginine-deficient mutants, each lacking a different enzyme necessary for synthesizing arginine

  6. 8. Class I Mutants Class II Mutants Class III Mutants Wild type Minimal Medium (MM) (control) MM + Ornithine MM + Citrulline MM + arginine (control) Class I Mutants (mutation In gene A) Class II Mutants (mutation In gene B) Class III Mutants (mutation In gene C) Wild type Precursor Precursor Precursor Precursor Enzyme A Gene A A A A Ornithine Ornithine Ornithine Ornithine Enzyme B Gene B B B B Citrulline Citrulline Citrulline Citrulline Enzyme C Gene C C C C Arginine Arginine Arginine Arginine

  7. 9. Beadle and Tatum’s experiment indicated that: • Biochemical pathways, such as those that produce amino acids, are arranged in a series of steps. Each step is catalyzed by an enzyme. • This experiment supported their “one gene-one enzyme” hypothesis

  8. 10. The Products of Gene Expression: A Developing Story • Some proteins aren’t enzymes, so researchers later revised the hypothesis: one gene–one protein • Many proteins are composed of several polypeptides, each of which has its own gene • Therefore, Beadle and Tatum’s hypothesis is now restated as “one gene–one polypeptide”

  9. 11. Overview: Basic Principles of Transcription and Translation • RNA is ______________; DNA is double-stranded • RNA contains the 5-carbon sugar _______________; DNA contains the 5-carbon sugar deoxyribose • RNA contains the nitrogenous base ______________; DNA contains the nitrogenous base thymine

  10. 12. Overview: Basic Principles of Transcription and Translation • The monomers of DNA and RNA are ________________________. • The monomers of proteins are __________________________.

  11. 13. Overview: Basic Principles of Transcription and Translation • Transcription is the synthesis of RNA under the direction of DNA • Transcription produces messenger RNA (mRNA) in the nucleus of eukaryotic cells • Translation is the synthesis of a polypeptide, which occurs under the direction of mRNA • Ribosomes are the sites of translation

  12. 13—14. • In prokaryotes, mRNA produced by transcription is immediately translated without more processing • In a eukaryotic cell, the nuclear envelope separates transcription from translation • Eukaryotic RNA transcripts are modified through RNA processing to yield finished mRNA • Cells are governed by a cellular chain of command: DNA RNA protein

  13. 13—14. DNA TRANSCRIPTION Prokaryotic cell

  14. 13—14. DNA TRANSCRIPTION mRNA Ribosome Prokaryotic cell Polypeptide Prokaryotic cell

  15. 13—14. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope DNA TRANSCRIPTION Eukaryotic cell

  16. 13—15. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Nuclear envelope In eukaryotes, the pre-mRNA is called a primary transcript DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Eukaryotic cell

  17. 13—16. DNA TRANSCRIPTION mRNA Ribosome TRANSLATION Polypeptide Prokaryotic cell Francis Crick’s Central Dogma: DNA RNA protein Nuclear envelope DNA TRANSCRIPTION Pre-mRNA RNA PROCESSING mRNA Ribosome TRANSLATION Polypeptide Eukaryotic cell

  18. 17—19. The Genetic Code • How are the instructions for assembling amino acids into proteins encoded into DNA? • There are 20 amino acids, but there are only four nucleotide bases in DNA • So how many bases correspond to an amino acid? • Would one nucleotide coding for one a.a. work? 41? • How about 2 nucleotides for one a.a.? 42? • How about 3? 43?

  19. 19. Codons: Triplets of Bases • The flow of information from gene to protein is based on a triplet code: a series of nonoverlapping, three-nucleotide words • These triplets are the smallest units of uniform length that can code for all the amino acids • Example: AGT at a particular position on a DNA strand results in the placement of the amino acid serine at the corresponding position of the polypeptide to be produced

  20. 20. • During transcription, a DNA strand called the template strand provides a template for ordering the sequence of nucleotides in an RNA transcript • During translation, the mRNA base triplets, called codons, are read in the 5 to 3 direction • Each codon specifies the amino acid to be placed at the corresponding position along a polypeptide

  21. 21—22. This is not the same code as in the F & T Guide! Gene 2 DNA molecule Gene 1 Gene 3 5¢ 3¢ DNA strand (template) TRANSCRIPTION 5¢ 3¢ mRNA Codon TRANSLATION Protein Amino acid

  22. 23—24, 28. Cracking the Code • All 64 codons were deciphered by the mid-1960s • Nirenberg made “synthetic poly-U” mRNA and exposed it to a yeast cell extract. The resulting a.a. sequence was phe-phe-phe-phe-phe… Therefore, UUU codes for phenylalanine • The genetic code is redundant but not ambiguous; no codon specifies more than one amino acid • Codons must be read in the correct reading frame (correct groupings) in order for the specified polypeptide to be produced

  23. 25—28. Second mRNA base First mRNA base (5¢ end) Third mRNA base (3¢ end)

  24. 29. Reading Frame • The reading frame for DNA is that each three “letters” in an mRNA codon codes for one “word,” which is one amino acid. That is how the code is deciphered. • When you are reading the sentences in this powerpoint, you are using a reading frame, too. In this case, each word is separated by a space. That’s how you know how to decipher this code (English).

  25. 30. Evolution of the Genetic Code • The genetic code is nearly universal, shared by the simplest bacteria to the most complex animals • Genes can be transcribed and translated after being transplanted from one species to another

  26. 30. This tobacco plant is expressing the gene for luciferase, the enzyme fireflies have which makes them glow

  27. Concept 17.2: Transcription is the DNA-directed synthesis of RNA: a closer look

  28. 31—32. Molecular Components of Transcription • RNA synthesis is catalyzed by RNA polymerase, which pries the DNA strands apart and hooks together the RNA nucleotides • RNA polymerase does not require a primer to synthesize a new strand of RNA • RNA synthesis follows the same base-pairing rules as DNA, except uracil substitutes for thymine

  29. 33. • The DNA sequence where RNA polymerase attaches is called the promoter; in prokaryotes, the sequence signaling the end of transcription is called the terminator • The stretch of DNA that is transcribed is called a transcription unit Animation: Transcription

  30. 34. Transcription unit Promoter 5¢ 3¢ 3¢ 5¢ DNA Start point RNA polymerase

  31. 34. Promoter Transcription unit 5¢ 3¢ 5¢ 3¢ DNA Start point RNA polymerase Initiation 5¢ 3¢ 3¢ 5¢ Template strand of DNA RNA tran- script Unwound DNA

  32. 34. Promoter Transcription unit 5¢ 3¢ 5 3¢ DNA Start point RNA polymerase Initiation 5¢ 3¢ 5 3¢ Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA 5¢ 3¢ 3¢ 5 3¢ 5¢ RNA transcript

  33. 34. Promoter Transcription unit 5 3 3¢ 5¢ DNA Start point RNA polymerase Initiation 5¢ 3¢ 5¢ 3¢ Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA 5¢ 3¢ 3¢ 3¢ 5¢ 5¢ RNA transcript Termination 5¢ 3¢ 3¢ 5¢ 5¢ 3¢ Completed RNA transcript

  34. 34. Elongation Non-template strand of DNA RNA nucleotides RNA polymerase 3¢ 3¢ end 5¢ Direction of transcription (“downstream”) 5¢ Template strand of DNA Newly made RNA

  35. Synthesis of an RNA Transcript • The three stages of transcription: • Initiation • Elongation • Termination

  36. 35, 38. RNA Polymerase Binding and Initiation of Transcription • Promoters signal the initiation of RNA synthesis • Transcription factors mediate the binding of RNA polymerase and initiation of transcription • The completed assembly of transcription factors and RNA polymerase II bound to a promoter is called a transcription initiation complex

  37. 36. Eukaryotic promoters Promoter 5¢ 3¢ 3¢ 5¢ TATA box Start point Template DNA strand Several transcription factors Transcription factors 5¢ 3¢ 3¢ 5¢ Additional transcription factors RNA polymerase II Transcription factors 5¢ 3¢ 5¢ 3¢ 5¢ RNA transcript Transcription initiation complex

  38. 37. • A part of the promoter, called a TATA box, is crucial in forming the initiation complex in eukaryotes

  39. 39. Whoa, there, Fred and Theresa! • You want my students to write an essay about transcription, but you haven’t covered elongation or termination yet! • Chill out!

  40. Elongation of the RNA Strand • As RNA polymerase moves along the DNA, it untwists the double helix, 10 to 20 bases at a time • Transcription progresses at a rate of 60 nucleotides per second in eukaryotes • A gene can be transcribed simultaneously by several RNA polymerases

  41. Termination of Transcription • The mechanisms of termination are different in prokaryotes and eukaryotes • In prokaryotes, the polymerase stops transcription at the end of the terminator • In eukaryotes, the polymerase continues transcription after the pre-mRNA is cleaved from the growing RNA chain; the polymerase eventually falls off the DNA

  42. 39. Okay, now write your essay… Promoter Transcription unit 5 3 3¢ 5¢ DNA Start point RNA polymerase Initiation 5¢ 3¢ 5¢ 3¢ Template strand of DNA RNA tran- script Unwound DNA Elongation Rewound DNA 5¢ 3¢ 3¢ 3¢ 5¢ 5¢ RNA transcript Termination 5¢ 3¢ 3¢ 5¢ 5¢ 3¢ Completed RNA transcript

  43. Concept 17.3: Eukaryotic cells modify RNA after transcription • Enzymes in the eukaryotic nucleus modify pre-mRNA before the genetic messages are dispatched to the cytoplasm • During RNA processing, both ends of the primary transcript are usually altered • Also, usually some interior parts of the molecule are cut out, and the other parts spliced together

  44. 40—41. Alteration of mRNA Ends • Each end of a pre-mRNA molecule is modified in a particular way: • The 5 end receives a modified nucleotide cap • The 3 end gets a poly-A tail • These modifications share several functions: • They seem to facilitate the export of mRNA • They protect mRNA from hydrolytic enzymes • They help ribosomes attach to the 5’ end

  45. 40—41. UTRs are untranslated regions, but function in ribosome binding Protein-coding segment Polyadenylation signal 5¢ Start codon Stop codon 3¢ UTR Cap 5¢ UTR 5¢ Poly-A tail

  46. 42. Split Genes and RNA Splicing • Most eukaryotic genes and their RNA transcripts have long noncoding stretches of nucleotides that lie between coding regions • These noncoding regions are called intervening sequences, or introns • The other regions are called exons because they are eventually expressed, usually translated into amino acid sequences • RNA splicing removes introns and joins exons, creating an mRNA molecule with a continuous coding sequence

  47. 43. 5¢ Exon Intron Exon Intron Exon 3¢ Pre-mRNA 5¢Cap Poly-A tail 1 30 31 104 105 146 Introns cut out and exons spliced together Coding segment Poly-A tail 5¢Cap 1 146 UTR 5¢ 3¢ UTR

  48. 44—45. • In some cases, RNA splicing is carried out by spliceosomes • Spliceosomes consist of a variety of proteins and several small nuclear ribonucleoproteins (snRNPs) that recognize the splice sites • snRNAs are short segments of RNA, about 150 nucleotides long

  49. 46—47. RNA transcript (pre-mRNA) 5¢ Exon 1 Intron Exon 2 Protein Other proteins snRNA snRNPs Spliceosome 5¢ Spliceosome components Cut-out intron mRNA 5¢ Exon 1 Exon 2

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